This invention pertains to the direct translation of injected microwave signals to optical signals.
The possibility of transmitting very high density information, e.g., voice, video or mathematical, via optical fibers has been recognized for some years. Such speculative schemes generally comprise the use of a microwave-driven laser diode for converting a modulating microwave signal in the frequency range of 5 GHz or higher to an optical signal consistent with the input microwave signal. However, it is now generally agreed that such high density communication will come about only if substantially deeper modulation of the optical signal can be achieved while avoiding or at least greatly suppressing the distortion characteristic of present day devices. The expression "modulation depth", as used herein refers to modulation of signal intensity as defined by the known expression, ##EQU1## wherein I is the intensity.
In early cryogenic lasers, modulation at X-band (i.e. 8-12 GHz) frequencies was demonstrated by Goldstein et al, as reported in Proc. IEEE 53 195 (1965). At about the same time Myers et al reported modulation at frequencies as high as 46 GHz, in J. Appl. Phys. 36 22 (1965).
More practical room temperature continuous wave lasers were reported in 1969. Microwave modulation in such lasers, for example by Gunn oscillators, IMPATT or TRAPATT diodes, has since been carried out in specially designed cavities. However, the transmission of high-density optical signals was not realized because in all such devices the achievable depth of modulation was only a few percent.
The basic arrangement for direct modulation of a laser diode, to which this invention pertains, normally comprises d.c. biasing to a point above the lasing threshold in order to avoid a sharp discontinuity in the output near the threshold. Above the threshold, response is essentially linear in a well made diode.
It has been recognized for some time that increased modulation depth would depend on the prevention of microwave leakage to the bias circuitry and to other components. The former object has been met by high-pass and low-pass circuits, as known in the art. Specially designed microwave cavities have been employed to shield these peripheral circuits against microwave leakage, while still providing the necessary coupling of the laser diode to the modulating signal. For example, Carroll et al, in Electron. Lett. 9 166 (1973), described a cross-shaped coaxial cavity to couple a TRAPATT modulator to a laser diode, obtaining 180 ps. pulses of light at 1 GHz repetition rate. Similarly, Lakshminarayana et al in Electron. Lett. 14 640 (1978), described a device wherein a laser diode was mounted in a low capacitance package placed in an X-band coaxial cavity. The diode was driven by a 100 mW microwave signal from a Gunn oscillator to produce modulation at 10.6 GHz.
It appears not to have been recognized that in these devices undesirable leakage of the microwave signal through the optical channel could occur. The leaking microwaves might and probably did couple with other components of the assembly such as the power supply, to produce distorted signals which were superposed on the information signal. In experimental devices reported in the art, microwaves leaking through the optical channel probably coupled with the nearby detector. Modulation depths of only a fraction of a percent could be achieved with these art devices.